Generally speaking you seem to be on the right track, but you really need someone to go through the construction drawings in detail, or perhaps visit the site, since it's an existing building that you are modifying. It's hard for us to get a full appreciation of all the factors involved via email. We also point out that we have no power to give building approval and it will be necessary sooner or later for you to engage a suitably qualified person to advise you. If your usual certifier is "too busy" we suggest you consult a fire engineer.

If your certifier is looking for the Group Number of a particular hardwood, several Regulatory Information Reports on our website show that all timbers tested so far fall into Group 3. For example Report Number RIR 41117.10, which can be downloaded here https://www.woodsolutions.com.au/fire-test-reports-1, includes fire properties for many commonly used timbers.